1. Field of the Invention
The present invention relates to a failure diagnosis device for an O2 sensor used in a fuel injection control system of an internal combustion engine.
2. Description of the Related Art
In a fuel injection control system of an internal combustion engine, an O2 sensor is used as air/fuel ratio detection means for detecting an air/fuel ratio in exhaust gas from an internal combustion engine. In order to determine whether the O2 sensor is operating normally, a determination is made by taking advantage of the fact that output voltage from the O2 sensor changes near the theoretical air/fuel ratio. Specifically, by making the engine operate under combustion conditions that would cause the exhaust gas to enter a rich state, it becomes possible to judge that the sensor is operating normally when the output from the O2 sensor changes from a lean state to the rich state.
However, in the O2 sensor failure diagnosis according to the conventional example described above, it is also necessary to have the engine actually running, and it is necessary that the engine run at an air/fuel ratio richer than the theoretical air/fuel ratio. Accordingly, exhaust gas is generated during the internal combustion engine manufacturing process, thus having a negative effect on the environment.